1. Field of the Invention
The present invention relates generally to fiber optical WDM transmission systems and optical amplifiers used therein, and more particularly to an optical feedback resonant laser cavity (OFRC), including a power dependent loss element (PDLE) for optical gain control (OGC) or optical power control (OPC), and to a method for implementing such control, which is particularly useful, although not so limited, in amplified wavelength add/drop multiplexed (WADM) transmission nodes.
2. Technical Background
Wavelength division multiplexing (WDM) is a demonstrated technology for increasing the capacity of existing fiber optic networks. A typical WDM system employs multiple optical signal channels, each channel being assigned a particular wavelength or wavelength band. In a WDM system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel is individually routed to a designated receiver. Multiple optical channels can be amplified simultaneously in optical amplifiers such as erbium doped fiber amplifiers (EDFAs), facilitating the use of WDM systems for long distance transmission.
Add-drop multiplexers are used, for instance, at nodes in a WDM communication network to extract one or more channels from the multiplexed stream, letting the remaining channels pass through unaltered to the next node, and to add to the multiplexed stream a new channel for transmission. Another application of such devices is for routing nodes of reconfigurable optical networks, namely rerouting certain information streams as a result of changed traffic conditions or to remedy a failure downstream from the node.
As schematically illustrated in FIG. 1. a conventional WADM node 120 consists respectively of gain-controlled input and output amplifiers 121, 123, a pair of 1xc3x97N and Nxc3x971 multiplexers/demultiplexers 125, 127, and an array of add/drop switches 129. This type of WADM node is herein referred to as a 1xc3x97Nxc3x971 node since there is a single input amplifier and a single output amplifier, both of which are likely gain flattened and gain controlled amplifiers. Wavelength add/drop multiplexing permits signals to be routed from different networks or propagate through different spans. As a result, however, the per channel power after each add/drop switch can vary significantly, say by YdB. An approach to equalize channel power at the output of each node is to monitor per channel power and to use a variable optical attenuator (VOA) 131 in each channel path to maintain constant channel power. Due to the characteristically slow VOA response, however, the settling time for channel adding varies from milliseconds to seconds depending upon the VOA technology employed. Although VOA response times are not currently problematic (as some traffic interruption is expected for switched channels), there is a significant pump power penalty in the output amplifier imposed by the required protection of the surviving channels. For example, a VOA with a feedback control has to level the channel power which may vary between channels by a value of YdB (that is, by 10(Y/10) times as much on a linear scale), as mentioned above. To protect the signal in a worst case where Nxc3x971 channels are added simultaneously. all with a YdB excess power, the pump power of the output amplifier needs to support (10(Y/10)xc3x97(Nxe2x88x921))+1 channels of power before the VOA can respond. In other words, the pump power penalty to protect the surviving channel is almost YdB if N is large.
One suggested approach to address this problem consists of replacing the VOAs and the output amplifier of the 1xc3x97Nxc3x971 architecture with multiple parallel power equalization amplifiers (PEAs) as schematically shown in FIG. 2, to form what is referred to herein as a 1xc3x97Nxc3x97N architecture because there are N optically controlled outputs. Each PEA can be designed to operate in its saturation regime so that the output signal power is determined by the pump power and is substantially independent of the input powers. Simulation results have shown that it is possible for the output power of these PEAs to differ by only 0.5 dB for an input power difference of 6 dB, and a ldB difference has been experimentally demonstrated. Although this approach is cost effective in that the VOAs and a complex output amplifier are eliminated from the system, and separate pump diodes for each PEA are replaced by a shared pump source, there is a recognized need for transient (as channels are added/dropped ) power control of each of the parallel amplifiers. Without such control the inversion of the amplifier is higher when a channel is dropped, and there is a large transient power spike when another channel is added back into the amplifier. Repeatedly amplifying this transient spike along the amplifier chain may result in component damage or an even greater pump power penalty to protect the surviving channels. However, because these parallel amplifiers share the pump source, individual transient control is not easily achieved by conventional electrical control such as by regulating the pump power.
One way of addressing this issue is to incorporate an optical feedback resonant laser cavity (OFRC) in each PEA so that the transient control is individually applied to these parallel amplifiers, even though they commonly share the pump power. For controlling the power transient of the PEAs, the OFRC is configured so that an optical power control (OPC) laser turns on (lases) when a signal channel is dropped from the PEA. Ideally, the OPC laser turns off (stops lasing) when a signal channel appears in the PEA, so that the PEA is only saturated by the signal channel and the signal channel extracts all of the available energy provided by the pump power. However, because there may be a power variation of YdB among the signal channels, the OPC laser has to be turned off by a signal channel having the lowest possible channel power. In order to achieve this operating condition, the cavity loss of the OFRC must be high. However, a high loss produces a low OPC laser power when the laser turns on, and thus a high amplifier inversion. As a result, if a signal channel with a high power is added into the highly inverted PEA which is saturated by an OPC laser with low power, there will be a transient power spike due to that high inversion. To eliminate the transient spike, the OFRC requires a lower cavity loss and higher OPC laser power. This presents the paradox of having a high loss and a low loss in the OFRC.
In addition to the transient power control of the PEAs, discussed above, the WDM input amplifiers shown in FIG. 1 and FIG. 2 are gain-controlled to reduce steady-state (DC) gain error. A common technique to implement such control in a WDM optical amplifier involves configuring each amplifier with an OFRC such as an optical gain control (OGC) laser cavity. It is well known for such a configuration that the optical gain must equal the passive loss at the lasing wavelength. As a result, for a homogeneous medium the optical gain at all wavelengths in a given spectrum is locked once the gain is fixed at any particular wavelength. Thus the gain spectrum of the amplifier is determined once the OGC laser wavelength and the passive loss at that wavelength are determined.
It is now appreciated by those skilled in the art that an erbium doped fiber (EDF) is not a purely homogeneous medium for light amplification; rather, it exhibits a certain degree of inhomogeneity. This circumstance gives rise to the phenomenon of spectral hole burning. When the power of the OGC laser is increased, for example, by dropping channels or by increasing the pump power, the spectral hole at the lasing wavelength gets deeper and results in a steady-state (DC) gain error in the signal band. This is illustrated schematically in FIG. 3(a). Accordingly, there is a need to solve the gain error problem caused by spectral hole burning by the OGC laser when signals are added or dropped, or the pump power is changed.
An embodiment of the present invention is directed to an optical amplifier that includes a gain medium; a pump source coupled thereto to excite the gain medium; an optical feedback resonant laser cavity (OFRC) coupled to the gain medium; and a power dependent loss element (PDLE) in the OFRC which exhibits a decreasing loss as a function of increasing incident laser power. The OFRC with a PDLE according to the invention provides optical gain control (OGC) for a WDM amplifier or optical power control (OPC) for a single channel amplifier, when either of which are subject to dynamically changing amplifier input conditions. In a preferred aspect of this embodiment, the PDLE is a passive mechanism such as a saturable absorber. The saturable absorber can be a length of rare earth doped fiber, and more preferably an erbium doped fiber. In an alternative aspect the PDLE can be an active mechanism such as a light intensity modulator, namely an acousto-optic modulator or an electrooptic modulator, with a feedback control. The preferable OFRC is in the form of a ring cavity, or alternatively a linear cavity. The OFRC structure and associated components used to couple the OFRC to the amplifier will substantially determine the lasing wavelength of the laser. For example, a ring cavity will preferably be coupled to the amplifier via two wavelength selective couplers which transmit the signal band wavelengths and couple the wavelength band of the OGC laser or OPC laser to the feedback cavity. Similarly, a linear cavity will preferably utilize a grating structure as a cavity end reflector/transmitter with the reflected light corresponding to the lasing wavelength and the transmitted light corresponding to the signal band wavelengths. The amplifier is preferably an EDFA, but alternatively could be a semiconductor amplifier. a Raman amplifier, a Brillouin amplifier, or other type of amplifier operating over conventional or extended optical bandwidths.
In another embodiment, a wavelength add/drop multiplexed (WADM) amplified optical transmission node includes an Minxc3x97Nout port demultiplexer with Minxe2x89xa71 and Nout greater than 1 for demultiplexing an optical signal having a wavelength range xcex94xcexin into a plurality of N optical signals each having a discreet wavelength xcexi (i=1 to N), or a plurality of N optical signals each having a wavelength range xcex94xcexj (j=1 to Nxe2x88x921); N add/drop signal propagation paths each coupled at one end thereof to an Nout port, wherein each signal propagation path includes an optical amplifier having a gain medium; and an Ninxc3x97Zout multiplexer with Zoutxe2x89xa71 for multiplexing at least some of the Nin optical signals, coupled to another end of the N add/drop signal propagation paths, and which is characterized in that a plurality of the N optical amplifiers each includes an optical feedback resonant laser cavity (OFRC) coupled to the respective gain media of the amplifiers and further includes a power dependent loss element (PDLE) that exhibits a decreasing loss as a function of increasing laser light intensity input to the PDLE and vice-versa.
As described above with regard to the optical amplifier embodiment, the preferable OGC or OPC laser cavity is in the form of a ring cavity, or alternatively a linear cavity, and the lasing wavelength is substantially determined by the coupled wavelength. For example, a ring cavity will preferably be coupled to the amplifier via two wavelength selective couplers which transmit the signal band wavelengths and couple the laser wavelength. Similarly, a linear cavity will preferably utilize a grating structure as a cavity end reflector/transmitter with the reflected light corresponding to the lasing wavelength and the transmitted light corresponding to the signal band wavelengths. In a preferred aspect of this embodiment, the PDLE again is a passive mechanism such as a saturable absorber. The saturable absorber can be a length of rare earth doped fiber, and more preferably an erbium doped fiber. In an alternative aspect, the PDLE similarly can be an active mechanism such as a light intensity modulator, namely, an acousto-optic modulator, with a feedback control.
In a further embodiment of the invention, a method for controlling a transient power change in a single channel optical amplifier or reducing a DC gain error in a WDM optical amplifier that are subject to dynamically variable operating conditions at an input of the amplifier, and including an OFRC coupled to a gain medium of the amplifier having an output power that is dynamically dependent upon the operating conditions, includes the step of decreasing the cavity loss of the OFRC as the output power of the OFRC increases or vice-versa, whereby the inversion of the amplifier gain medium is dynamically varied to reduce gain or power changes in the amplifier.
The invention described herein provides a device and method for improved optical gain or power control. The advantages of optical (in contrast to electronic) control are well appreciated and include the attributes of being passive (that is, substantially independent of gain ripple, signal input powers, and pump power) and self-contained. The invention is especially useful in WADM applications, as well as for soliton propagation systems, in which precise channel powers are important, and which benefit from the power xe2x80x9cre-equalizationxe2x80x9d at each 1xc3x97Nxc3x97N node. Since at each 1xc3x97Nxc3x97N node the channel power is re-equalized after a PEA such that the output power is independent of the input power, gain ripple does not accumulate along a chain of amplifiers.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed:
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention. and together with the description serve to explain the principles and operation of the invention.